Transmission apparatus and method for MIMO system

This application claims the benefit under 35 U.S.C. § 119(a) of an application entitled “Transmission Scheme for MIMO System” filed in the Korean Intellectual Property Office on Feb. 2, 2005 and assigned Serial No. 2005-9561, the entire contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates generally to a wireless communication system, and in particular, to a transmission scheme for a Multiple Input Multiple Output (MIMO) communication system.

2. Description of the Related Art

In a MIMO communication system that transmits/receives data using a plurality of transmission antennas and reception antennas, a MIMO channel formed by the transmission and reception antennas is disassembled into a plurality of independent spatial channels. Each of the spatial channels is mapped to one dimension, and the use of additional dimensions created by the multiple transmission and reception antennas improves performance of the MIMO system.

In a wireless communication system, transmission data forms a symbol stream through a coding and modulation process, and the symbol stream is phase-shifted into a radio frequency (RF) modulated signal that is suitable to be transmitted over a wireless channel.

A data transmission scheme using the MIMO system is classified into a Spatial Multiplexing (SM) scheme and a Spatial Diversity (SD) scheme. The SM scheme simultaneously transmits different data using multiple transmission antennas, thereby transmitting data at a high rate without increasing a bandwidth of the system. The SD scheme transmits single data stream via multiple transmission antennas, thereby obtaining transmit diversity gain.

FIG. 1 is a block diagram illustrating a structure of a transmitter based on a Bell Labs LAyered Space-Time (BLAST) scheme, which is the conventional SM scheme. An input bit stream is multiplexed into four parallel bit streams S 1 , S 2 , S 3 and S 4 by a multiplexer (MUX) 101 . The individual bit streams S 1 , S 2 , S 3 and S 4 are modulated into modulation symbols X 1 , X 2 , X 3 and X 4 according to a modulation technique of their associated modulators 103 , and then transmitted through four associated transmission antennas. In this case, the modulation technique is a quadrature phase shift keying (QPSK) scheme. In the BLAST scheme, multiplexing gain is in proportion to the number of transmission antennas and reception antennas. However, the BLAST scheme, as it provides no diversity gain, there is a decrease in bit error rate (BER) performance in a high signal-to-noise ratio (SNR) environment, and it suffers an abrupt change in performance depending on the type of receiver and transmission optimization technique.

FIG. 2 is a block diagram illustrating a structure of a transmitter based on a Double Space-Time Transmit Diversity (DSTTD) scheme, which is the conventional SD scheme. Modulation symbols X 1 , X 2 , X 3 and X 4 modulated through the process of FIG. 1 are pre-coded by a transmission matrix of a pre-encoder 105 before being transmitted. The transmission matrix has a form defined by Equation (1):

The DSTTD scheme using the 4×4 transmission matrix shown in Equation 1 has both a multiplexing gain of 2 and a diversity gain of 2, thus contributing to performance improvement, but results in a decrease in data rate due to the reduction in the multiplexing gain.

For improvement in system performance, the MIMO system has recently been designed so as to support at least one of the SM techniques and the SD techniques. In this scheme, a particular transmission technique is selected in a given time period according to channel conditions and required results.

However, because a transmission technique for spatial multiplexing and a transmission technique for spatial diversity are different from each other in design, supporting both of the transmission techniques for spatial multiplexing and spatial diversity within one system causes an increase in complexity of transceivers.

The present invention is provided to substantially solve at least the above problems and/or disadvantages. It is, therefore, an object of the present invention to provide a transmission apparatus and method for improving multiplexing gain while maintaining the diversity gain available in the conventional spatial diversity technique, through a simple coding process.

It is another object of the present invention to provide a transmission apparatus and method for adjusting diversity gain through modification of a bit/symbol mapping pattern.

It is further another object of the present invention to provide a transmission apparatus and method for improving multiplexing gain and adjusting diversity gain without increasing complexity of a decoder.

It is still another object of the present invention to provide a transmission apparatus and method for maximizing system performance by adjusting multiplexing gain and diversity gain according to environment, without increasing complexity.

According to one aspect of the present invention, there is provided a transmitter in a multiple input multiple output (MIMO) communication system. The transmitter includes a multiplexer for multiplexing an input bit stream into a plurality of bit streams, and outputting the bit streams in parallel; a bit-order rearranger for reordering bits constituting the bit streams, which are output from the multiplexer in the same time period, and outputting the reordered bits as reordered bit streams; and a modulator for modulating the parallel reordered bit streams output from the bit-order rearranger according to their associated modulation schemes, and transmitting the modulated symbols via their associated antennas.

According to another aspect of the present invention, there is provided a transmission method in a multiple input multiple output (MIMO) communication system. The method includes multiplexing a serial input bit stream into parallel symbols, which are output in the same time period; generating reordered bit streams by reordering bits constituting the parallel output symbols; and modulating the reordered bit streams according to their associated modulation schemes, and simultaneously transmitting the modulated symbols via their associated antennas.

According to further another aspect of the present invention, there is provided a transmission method in a multiple input multiple output (MIMO) communication system. The method includes multiplexing a serial input bit stream and outputting four symbols in the same time period; mapping the four symbols in units of two symbols; generating four reordered bit streams by reordering bits constituting the reordered bit streams; and modulating the generated four reordered bit streams according to their associated modulation schemes and simultaneously transmitting the modulated symbols via their associated transmission antennas.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram illustrating a structure of a transmitter based on a Bell Labs Layered Space-Time (BLAST) scheme which is the conventional spatial multiplexing scheme;

FIG. 2 is a block diagram illustrating a structure of a transmitter based on a Double Space-Time Transmit Diversity (DSTTD) scheme which is the conventional spatial diversity scheme; and

FIG. 3 is a block diagram illustrating a structure of a Multiple Input Multiple Output (MIMO) transmitter according to the present invention.

A MIMO transmission apparatus and method according to the present invention will now be described with reference to the accompanying drawings.

FIG. 3 is a block diagram illustrating a structure of a MIMO transmission apparatus according to the present invention. As illustrated in FIG. 3 , a MIMO transmission apparatus according to the present invention includes a multiplexer 301 for multiplexing an input bit stream b 1 , b 2 , b 3 , b 4 , . . . into a plurality of parallel bit streams S 1 , S 2 , S 3 , and S 4 , a bit-order rearranger 303 for reordering bits constituting the parallel bit streams into a plurality of reordered bit streams M 1 , M 2 , M 3 , and M 4 , and a plurality of modulation modules 305 for modulating the reordered bit streams M 1 , M 2 , M 3 , and M 4 into transmission symbols X 1 , X 2 , X 3 and X 4 using a predetermined modulation technique and transmitting the transmission symbols X 1 , X 2 , X 3 and X 4 via their associated transmission antennas. The transmission symbols X 1 , X 2 , X 3 and X 4 may be pre-coded through a transmission matrix before being transmitted. Preferably, the modulation technique utilized by the present invention is a 16-ary Quadrature Amplitude Modulation (16 QAM) scheme.

Herein, it will be assumed for illustration purposes that the number Nt of transmission antennas is 4 (N t =4).

Upon receiving the input bit stream b 1 , b 2 , b 3 , b 4 , . . . , the multiplexer 301 generates four parallel bit streams S 1 , S 2 , S 3 and S 4 by processing the input bit stream b 1 , b 2 , b 3 , b 4 , . . . in units of 8 bits. Each of the generated symbols includes two bits. In other words, bit stream S 1 represents input bits b 1 and b 2 , bit stream S 2 represents input bits b 3 and b 4 , bit stream S 3 represents input bits b 5 and b 6 , and bit stream S 4 represents input bits b 7 and b 8 . The generated bit streams S 1 , S 2 , S 3 and S 4 are input in parallel to the bit-order rearranger 303 , and the bit-order rearranger 303 generates reordered bit streams M 1 , M 2 , M 3 and M 4 , the number of which is equal to the number of its input bit streams S 1 , S 2 , S 3 and S 4 , by performing mapped-reordering on the bits constituting the bit streams S 1 , S 2 , S 3 and S 4 . In the bit reordering process, a first reordered bit stream M 1 is configured by sequentially ordering the input bits b 1 and b 2 constituting the first bit stream S 1 and the input bits b 5 and b 6 constituting the third bit stream S 3 . A second reordered bit stream M 2 is configured by sequentially ordering the input bits b 3 and b 4 constituting the second bit stream S 2 and the input bits b 7 and b 8 constituting the fourth bit stream S 4 . Similarly, a third reordered bit stream M 3 is configured by sequentially ordering the input bits b 5 and b 6 constituting the third bit stream S 3 and the input bits b 1 and b 2 constituting the first bit stream S 1 , and a fourth reordered bit stream M 4 is configured by sequentially ordering the input bits b 7 and b 8 constituting the fourth bit stream S 4 and the input bits b 3 and b 4 constituting the second bit stream S 2 . In conclusion, the reordered bit streams M 1 , M 2 , M 3 and M 4 have bit streams of M 1 =b 1 b 2 b 5 b 6 , M 2 =b 3 b 4 b 7 b 8 , M 3 =b 5 b 6 b 1 b 2 , and M 4 =b 7 b 8 b 3 b 4 , respectively. The configured reordered bit streams M 1 , M 2 , M 3 and M 4 are input to the modulator 305 , and modulation modules constituting the modulator 305 modulate the reordered bit streams M 1 , M 2 , M 3 and M 4 into transmission symbols X 1 , X 2 , X 3 and X 4 using a 16-ary Quadrature Amplitude Modulation (16 QAM) scheme and transmit the transmission symbols X 1 , X 2 , X 3 and X 4 via their associated transmission antennas.

Although each of the reordered bit streams is created herein by sequentially ordering bits constituting two different bit streams, the number of bits constituting a bit stream, the number of bits copied from a bit stream, the number of bits constituting a reordered bit stream, the number of bit streams, from each of which bits constituting a reordered bit stream are to be copied, and the ordering sequence of the copied bit streams are subject to change according to system environment, and the change causes a change in diversity gain and data rate.

In addition, although 16 QAM is used herein as a modulation scheme, the embodiment of the present invention is not limited to this, and can also be applied to various modulation schemes, such as Quadrature Phase Shift Keying (QPSK) or 64 QAM.

As can be understood from the foregoing description, the novel data transmission apparatus and method can improve diversity gain while maintaining a data rate available in the conventional spatial multiplexing transmission scheme and can also increase a data rate while maintaining diversity gain available in the conventional spatial diversity transmission scheme, through a simple coding process using bit-order rearranging.

In addition, the novel data transmission apparatus and method can adjust multiplexing gain and diversity gain according to required system conditions without increasing system implementation complexity, thereby maximizing system performance.

While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.